I enjoy learning how to modify files with awk, sed, and bash. However, I know it could cause issues down the road and I would like to be prepared with a log inside the file. I was thinking about how I could create a log in the footer after I modify an .html file but I am unsure how or what the process is called. I would like to be able to do as follows:

line break after the very last line of code
comment tag <!--
each item modifed on a line
end comment tag -->

You need to be more specific about what exactly you are trying to do. If you just want revision control there are many existing tools that already exist for that, although some initial learning curve overhead might be required.
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jw013Dec 20 '12 at 19:04

As @ChrisDown has said, just go with a version control system. No need to reinvent existing, sophisticated wheels crudely (it takes a lot of time to implement an efficient and robust VCS).
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jw013Dec 20 '12 at 19:23

I didn't realize I needed a VCS to detect the last line. I think LOG may be a bad choice to use in this case but I am unsure on how to identify what I am trying to do. If I can figure out how to detect the last line and skip a line I can achieve what I want.
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Darth_VaderDec 20 '12 at 19:25